My Theory (possible spoilers)
There are aspects of each "world" that provide strong evidence that they aren't reality.
Sepia world (what we are meant to assume is real life near the start of the film): People are motionless and a dog disappears. I suppose some sort of mental illness could explain a "disappearing" dog but why are there people who seem to be frozen.
Game world: People pixelate and disappear when they are killed. Ash is shown logging in and logging out of the virtual reality by putting on some sort of a helmet.
Class real world: Murphy pixelates and disappears when he is killed.
I would say that the sepia and class real worlds are different versions of the same world as Ash exists in both of them after removing the helmet and seemingly disconnecting from the game. I can only assume that if you exist in a place before you put a virtual reality helmet on, then once you remove that helmet you are back in the same place. Since this place is not reality it is just programming that makes "class real" more colorful and vibrant but it is still the same non reality world as the sepia world. Also Ash sees her dog in the sepia world and then it disappears. The next time she sees it is in the class real world, albeit in a different form. Its just another example that the two are the same world and programming has changed her dog from her pet to a poster/dog riding in a strangers car. A third example is that Ash sees Murphy as a vegetable in the sepia world and then as a seemingly healthy man in the class real world.
Since the rule in this movie for determining if a world is reality seems to be watching someone die and seeing if they disappear then IMO we don't ever see reality since in both the game world and the sepia/class real world people disappear when they are killed.